1. Field of the Invention
This invention relates to the field of automatic detectors for letterbox video sources, for example as would be useful in wide screen televisions.
2. Description of the Art
In "letterbox" format, an image in wide format display ratio, for example 16.times.9 (also designated 16:9), is displayed across the full width of a narrower format display ratio medium, for example 4.times.3 (also designated 4:3). Movies in a wide format to be broadcast commercially without distortion or cropping are converted by the broadcaster to letterbox format, to enable display on a conventional receiver screen, for example 4.times.3. The broadcaster effectively adds blank or matte top and bottom borders, and broadcasts the combined picture and borders in a 4.times.3 composite image signal. Assuming standard 16.times.9 and 4.times.3 ratios, and standard NTSC signals, only 181 horizontal lines in each field are devoted to the main video, the remaining lines being the matte, gray or black borders. The useful picture information, or main video, is generally referred to as active video, whereas the borders are generally referred to as inactive video, even if the borders are flat fields having some luminarice value and/or chrominance component. This distinction and designation is followed herein. The borders may be used, if at all, to display different types of auxiliary information. A first type is generally text, such as storm warnings, news alerts and subtitles. A second type includes station and network logos, which are often graphic or a combination of text and graphics. Both types of auxiliary information might be present together, or separately. Both types of auxiliary information are also generally referred to as active video. A third type of auxiliary information which can interfere with correct identification of a signal in letterbox format is closed captioning, which is usually transmitted during the vertical blanking interval.
When such letterbox displays (in 4.times.3 format) are shown on a wide screen television, for example 16.times.9 format, the video signal can be vertically zoomed, that is enlarged, by a factor which both increases the portion of active video to substantially fill the wide screen display, and at the same time, substantially eliminates the upper and lower borders. This can be accomplished without incurring image aspect ratio distortion. It is a useful feature to automatically detect letterbox sources and vertically enlarge the active portion of the video source signal so as to substantially fill the display screen.
An automatic letterbox detector in a wide screen television is disclosed in PCT/US91/03739 (WO 91/19390). As described therein, the uppermost and lowermost active video areas are detected by calculating two gradients for each line in the video field. Four values are required to calculate the two gradients: maximum and minimum values of the current line, and maximum and minimum values of the previous line. The first gradient, designated the positive gradient, is formed by subtracting the minimum value of the previous line from the maximum value of the current line. The second gradient, designated the negative gradient, is formed by subtracting the minimum value of the current line from the maximum value of the previous line. Either of the gradients may have positive or negative values depending on scene content, but the negative values of both gradients may be ignored. This is because only one gradient may be negative at a time, and the magnitude of the gradient with the positive value will always be greater than or equal to the magnitude of the gradient with the negative value. This simplifies the circuitry by eliminating the need to calculate an absolute value of the gradients. If either gradient has a positive value which exceeds a programmable threshold, video is considered to be present on either the current line or on the previous line.
These values can be used by a microprocessor to make a determination of whether or not the video source is in the letterbox format and to determine the first and last line numbers of active video. The calculation can be performed only for the top (or the bottom) of the image, provided means are provided for centering the image vertically. Inasmuch as the detection of active video is a function of gradient rather than luma absolute value, the device can detect inactive borders of a predetermined matte color, provided the borders have a substantial (but unchanging) luminance level. The luminance level detector is enabled for only about 70% of each line in order to improve signal to noise ratios.
However, if the borders are used to display auxiliary material, such as storm warnings, news alerts, subtitles, station or network logos, and the like, the automatic letterbox detector will return operation to the standard, non-zoomed operation. This may be desirable for storm warnings, news alerts and subtitles, but can be a significant nuisance for logos. Moreover, continuous transmission of a logo in the border area of a letterbox source can prevent automatic detection of the source as being letterbox in the first instance.
Interference with detection of signals in letterbox format can also be caused by any VCR of the helical scan type, for example video cassette recorders utilizing the VHS format, which will insert a head switching transient anywhere from 5-8 lines prior to the vertical synchronizing signal. Head switching transients can be mistaken for active video, causing incorrect letterbox detection. False detections based on VCR head switching transients must be avoided.